In the photometric measurement of the light absorption of a liquid sample which is contained in a cuvette, the light beam exiting from the cuvette is measured by means of a photocell. Within a given range of light intensity, the voltage given off by the cell is approximately a linear function of the intensity of the light incident upon the photocell. However, the range of light intensity over which the cell exhibits such a linear response is limited and is very much smaller than the range over which light absorption can vary. This is allowed for by carrying out the measurements at different path lengths, a longer path length being chosen if the absorbance of the sample is small and a shorter path length being chosen if the absorbance of the sample is large. It is known (German Offenlegungsschrift No. 28 10 117) to carry out such measurements by using in a vertical position an elongated cuvette which has, distributed along its length, regions with different cross-section lengths and different thicknesses of the cuvette wall, the cross-section length and the wall thickness changing stepwise. In this known method, the cuvette is raised or lowered to move the region of whichever path length is desired into the path of the rays between the light source and the photocell.
Since in the known method it is necessary to raise or lower individual cuvettes in order to change the measured path length, the method is not suitable for the rapid analyses of samples in a large number of cuvettes. Moreover, the form of the cuvette used in the known method makes necessary a relatively large volume of sample. Since the thickness of the cuvette wall and hence the resulting light absorption differs in the various regions of the known cuvette, this difference must be taken into consideration when determining the result of a measurement, which naturally constitutes an additional complication.
It is the main object of the present invention to provide a method of the type mentioned hereinbefore which is not accompanied by the aforementioned limitation in use and the aforementioned disadvantages and which thus makes possible, in particular, rapid analyses of samples in a large number of cuvettes, with small amounts of sample, and that in as simple a manner as possible. It is a further object of the invention to provide a cuvette which is suitable for carrying out such a method.